The present invention relates to the print head of an electrophotographic printer, more particularly to a print head employing light-emitting diodes as light sources.
In the electrophotographic printing process, the surface of a uniformly charged photosensitive drum is illuminated to form a latent electrostatic image. This image is developed to create a toner image, which is then transferred to a sheet of paper. A light-emitting-diode (LED) print head uses a linear array of LEDs to provide the necessary illumination of the photosensitive drum.
An LED print head comprises an LED unit including the linear array of LEDs, a lens array that focuses the light emitted by the LEDs onto the photosensitive drum, and a lens holder that holds the lens array and the LED unit. The LED unit comprises a printed circuit board, a plurality of LED array chips, and a plurality of integrated-circuit chips that drive the LED array chips. The LED array chips are mounted on the surface of the printed circuit board, the undersides of the LED array chips being bonded directly to a conductive ground pattern exposed on the surface of the printed circuit board.
For reasons of economy, the LED array chips are conventionally made as narrow as possible. The LEDs are therefore located near one edge of each chip. Furthermore, since the ground pattern must conduct return current from all the LED array chips with negligible voltage drop, the ground pattern must be comparatively wide. The ground pattern is conventionally wider than the LED array chips, and the highly reflective surface of the ground pattern is partly exposed around the edges of the LED array chips.
This creates a problem. Although the LEDs are basically surface-emitting devices, since they are disposed near the edges of the LED array chips, some of the emitted light leaks through the edges of the chips, and enters the lens array after being reflected from the exposed surface of the ground pattern. If the reflected light is sufficiently strong, it can create printing blemishes and reduce the resolution of the print head. A more detailed description of this problem will be given below.
An object of the present invention is to improve printing quality by reducing unwanted reflection of light in an LED print head.
Another object of the invention is to improve printing quality by assuring level mounting of the LED array chips in an LED print head.
The invented LED print head has at least one LED array chip with an array of light-emitting diodes formed near one edge. The diodes emit light through the surface of the LED array chip, but some of the light also leaks through this edge of the LED array chip.
The LED array chip is mounted on the component surface of a printed circuit board. The component surface has a first region, in which the LED array chip is mounted, and a second region adjacent to the first region. The second region extends up to the above-mentioned edge of the LED array chip, and is adjacent to all parts of this edge.
The printed circuit board has a conductive pattern to which the LED array chip is electrically coupled and mechanically bonded in the first region of the component surface.
The printed circuit board also has a material exposed on the component surface in all parts of the second region. This material is less optically reflective than the conductive pattern, and absorbs at least some of the light that leaks through the edge of the LED array chip.
The exposed material is, for example, a dielectric coating that protects the component surface of the printed circuit board. This coating covers all of the second region, covers part of the first region, supports the above-mentioned edge of the LED array chip, and at least partly supports the opposite edge of the LED array chip.
Alternatively, the material exposed in the second region is a conductive paste that covers both the first and second regions, mechanically and electrically coupling the LED array chip to the conductive pattern.
As another alternative, the material exposed in the second region may be the dielectric substrate of the printed circuit board.
In all three of these cases, the comparatively low optical reflectivity of the exposed material reduces reflection of light from the second region.
Level mounting is maintained because the LED array chip is supported either on the level surface of the conductive pattern, or on the level surface of the dielectric coating.